AQA Computer Science A-Level 4.10 Fundamentals of databases

Advanced Notes

www.pmt.education

Specification: 4.10.1 Conceptual data models and entity relationship modelling:

Produce a data model from given data requirements for a simple scenario involving multiple entities

Produce entity relationship diagrams representing a data model and entity descriptions in the form: Entity1 (Attribute1, Attribute2, .... )

4.10.2 Relational databases:

Explain the concept of a relational database Be able to define the terms:

● attribute ● primary key ● composite primary key ● foreign key

4.10.3 Database design and normalisation techniques:

Normalise relations to third normal form Understand why databases are normalised

4.10.4 Structured Query Language (SQL):

Be able to use SQL to retrieve, update, insert and delete data from multiple tables of a relational database

Be able to use SQL to define a database table 4.10.5 Client server databases:

Know that a client server database system provides simultaneous access to the database for multiple clients

Know how concurrent access can be controlled to preserve the integrity of the database

www.pmt.education

Data models When creating a database, you might be given data requirements from which you need to produce a data model: an abstract model of which things to store and what information about them should be recorded. Entities and attributes In database design, an entity is a thing about which data is to be stored, for example: a customer. Attributes are characteristics or other information about entities, for example: the customer’s name or address. Databases are formed of tables which are used to store multiple entities. Each entity usually has its own row in a table and fields of that row hold the entity’s attributes.

Table: Customers

CustomerID CustomerName CustomerAddress CustomerEmail

005  Shadow H. 310 St Anne's Rd, Blackpool FY4 2QP

shadow.t.h@sonic.net

006  S. Holmes 221b Baker St, Marylebone, London NW1 6XE

sh@arthurconandoyle.com

007  J. Bond 30 Wellington Square, Chelsea, London SW3 4NR

j.bond@sis.gov.uk

The table above stores information about a company’s customers. Each row in the table holds information about one customer (one entity). The fields in each row hold information about the customers (attributes) such as their name and address. Entity identifiers When creating a database, it’s important to ensure that each entity has an unique identifier . An entity identifier is an attribute given to each entity which is unique within that table. In the example above, CustomerID is most likely to be a suitable entity identifier. Some database tables have multiple attributes which are combined to form the table’s entity identifier.

www.pmt.education

Entity description When describing how information about an entity is to be stored in a database, an entity description can be used.

Customer (CustomerID, CustomerName, CustomerAddress, CustomerEmail)  The entity description above describes how information about customers is stored in the database table above. The name of the table is shown outside of brackets which contain each of the entity’s attributes separated by commas. Underlining can be used to identify the attribute or attributes which form the table’s entity identifier. In this case, the attribute CustomerID has been underlined.

Relational Databases

The tables in a database can be related to each other, linked by common attributes. There are three possible degrees of relationship between tables in a database: one-to-one, many-to-many and one-to-many. Entity relationship diagrams Entity relationship diagrams (or ER diagrams) are used to graphically represent the relationships between tables in a database. Tables are shown as rectangles and are joined by lines which can represent different types of relationship.

One-to-one

Many-to-many

one-to-many

Each car has one owner, and each owner has one car (this example assumes that nobody owns multiple cars).

Each car has many passengers. Each passenger sits in one car.

Each driver can drive many different cars. Each car is driven by many different drivers.

www.pmt.education

Primary and foreign keys A primary key is an attribute that provides an unique identifier for every entity in a database table. When tables are linked by a shared attribute, the attribute must be a primary key in one table and is called a foreign key in the other. A foreign key is an attribute in a table which is the primary key in another , related, table. If it is not possible to form a primary key from just one attribute, it is possible to combine attributes to form what is called a composite primary key . Example: The following database tables are related to one another.

Table: Flights

FlightNo 🔑 PilotNo 🗝 Destination

ESY8876  65587 Paphos

RYN4133  13584 Dublin

BRI1101  20547 Munich

ESY5655  65587 Edinburgh

BRI8989  20547 Athens

Table: Pilots

PilotNo 🔑 PilotName

65587 Adam Triston

13584 Charlotte Green

20547 Orville Wright The primary key in Pilots is PilotNo and is FlightNo in Flights. The tables are linked by the shared attribute PilotNo. This makes PilotNo a foreign key in Flights. The relationship in this example is one-to-many . Many different flight routes are operated by the same pilot.

www.pmt.education

Many-to-many relationships When linking many-to-many relationships, a new table has to be created. This new table is called a link table . The following example features the two tables Products and Customer. There is a many-to-many relationship here as many different types of products are each bought by many different customers.

Table: Products

ProductID 🔑 ProductName ProductPrice

155484765  Knife £18.99

233145882  Rope £4.45

366584554  Revolver £124.98

Table: Customers

CustomerName CustomerID 🔑

Professor Plum 155484765 

Miss Scarlet 233145882 

Reverend Green 233145882 

In order to model the relationship between the tables, a new table called Orders has to be created. This is a link table.

Table: Orders

OrderID 🔑 ProductID CustomerID

223 155484765 155484765 

223 233145882 155484765 

224 233145882 233145882 

225 233145882 233145882 

225 366584554 233145882 

www.pmt.education

Database Normalisation

Databases are normalised so that they can be efficient without any compromise to the integrity of their data . Normalising databases involves ensuring that entities contain no redundant or repeated data . A database that has been normalised allows for faster searching and sorting that an unnormalised database thanks to the smaller tables created in the normalisation process. Furthermore, normalised databases are easier to maintain than their unnormalised counterparts. Duplication of data is minimised and data consistency is improved, helping to reduce the number of update, insertion and deletion anomalies that occur. There are three levels of normalisation that you need to know: first, second and third normal form. First normal form When a database conforms to first normal form, it contains no repeating attributes . The database’s data can be referred to as atomic (meaning that no single column contains more than one value). This table contains repeating attributes so is not normalised to first normal form.

Table: Staff

Name 🔑 Department Subject 🔑 DepartmentHead

John Strode Earth Sciences Geography Jackie Smith

Sarah Ng Science Chemistry Brian Jones

Mary Marsh Science Physics, Biology Brian Jones Splitting the repeating attributes means that this database is now in first normal form.

Table: Staff

Name 🔑 Department Subject 🔑 DepartmentHead

John Strode Earth Sciences Geography Jackie Smith

Sarah Ng Science Chemistry Brian Jones

Mary Marsh Science Physics Brian Jones

Mary Marsh Science Biology Brian Jones

www.pmt.education

Second normal form In order to meet second normal form, a database must also satisfy first normal form. In second normal form, partial key dependencies are removed. A partial key dependency occurs in databases with composite primary keys when a non-key attribute doesn’t depend on the whole of the composite key . In our example, the primary key is composite.

Staff (Name, Department, Subject, DepartmentHead) 

Because the attribute DepartmentHead depends only on the attribute Department and Department depends only on the attribute Subject, the tables must be modified to meet second normal form.

Table: Staff

Name 🔑 Subject 🔑

John Strode Geography

Sarah Ng Chemistry

Mary Marsh Physics

Mary Marsh Biology

Table: SubjectDepartments Table: HeadsOfDepartment

Subject 🔑 Department Department 🔑 Head

Biology Science Science Brian Jones

Chemistry Science Earth Sciences Jackie Smith

Geography Earth Sciences

Physics Science Creating the two tables SubjectDepartments and HeadsOfDepartment has ensured that the database now conforms to second normal form as the partial key dependencies of Department and DepartmentHead have been removed from the Staff table.

www.pmt.education

Third normal form In order to meet third normal form, in addition to conforming to second normal form, a database must have no non-key dependencies. A database that meets third normal form can be described as follows:

All non-key attributes depend on the key, the whole key and nothing but the key

Our example meets third normal form as none of the attributes that do not form the key (or part of a composite key) depend on the anything other than the whole key.

Structured Query Language (SQL) SQL is a language used with databases. SQL is easy to learn and use, partly because it is a declarative language, meaning that the programmer describes the result that’s required rather than describing the process which should be followed. There are four main SQL commands: SELECT, UPDATE, INSERT and DELETE. The SELECT command SELECT is used for retrieving data from a database table. Commands take the following form: SELECT <attribute> FROM <table> WHERE <condition> ORDER BY <ASC/DESC> 

Note that the ORDER BY clause is optional. Let’s use the following table as an example.

Table: Flights

FlightNo 🔑 PilotNo Destination

ESY8876  13584 Glasgow

ESY1225  13584 Swansea

BRI1101  20547 Berlin SELECT FlightNo FROM Flights WHERE Destination = ‘Berlin’ >> BRI1101  SELECT Destination FROM Flights WHERE PilotNo = ‘13584’ ORDER BY FlightNo DESC >> Glasgow, Swansea  

www.pmt.education

The UPDATE command This command is used in databases for modifying the attributes of an existing entity and takes the form:

UPDATE <table> SET <attribute> = <value> WHERE <attribute> = <value> 

Table: Students

StudentNo 🔑 Name Email Year

55685  Aaron Aaronson a.a.aaronson@outlook.com 1

55887  Beth Hunter elisabeth.h@gmail.com 2

55622  Sam Cooper samc00per@hotmail.com 1 UPDATE Students SET Email = ‘beth24@yahoo.co.uk’ WHERE StudentNo = 55887 

UPDATE Students SET Name = Samuel Cooper WHERE StudentNo = 55622  Once the two UPDATE commands above have been carried out on the table above, the table looks like this:

Table: Students

StudentNo 🔑 Name Email Year

55685  Aaron Aaronson a.a.aaronson@outlook.com 1

55887  Beth Hunter beth24@yahoo.co.uk 2

55622  Samuel Cooper samc00per@hotmail.com 1 UPDATE commands usually use the table’s primary key to identify which entities to update but can use more general conditions which would update all of the entities that meet the condition.

UPDATE Students SET Year = 2 WHERE StudentNO < 55700  

Table: Students

StudentNo 🔑 Name Email Year

55685  Aaron Aaronson a.a.aaronson@outlook.com 2

55887  Beth Hunter beth24@yahoo.co.uk 2

55622  Samuel Cooper samc00per@hotmail.com 2

www.pmt.education

The DELETE command As you might expect, the DELETE command is used for removing entities from a database. The commands take the following form:

DELETE FROM <table> WHERE <condition>  

Table: Cars

Model 🔑 Manufacturer 🔑 Price Year Sold

Polo  Volkswagen 4995 2010 TRUE

i10  Hyundai 5225 2013 FALSE

Fiesta  Ford 3995 2009 TRUE

DELETE FROM Cars WHERE Sold = TRUE  

Table: Cars

Model 🔑 Manufacturer 🔑 Price Year Sold

i10  Hyundai 5225 2013 FALSE The INSERT command When using SQL to add new records to an existing table, the INSERT command is used. The command usually takes the form

INSERT INTO <table> (<column1>, <column2>, …) VALUES (<value1>, <value2>, …) but can be simplified to

INSERT INTO <table> VALUES (<value1>, <value2>, …) 

when all of the columns in the table are being used in the correct order. For example, executing the following commands would add two new records to the Cars table.

INSERT INTO Cars VALUES (“KA”, “Ford”, 3999, 2010, FALSE) INSERT INTO Cars (Model, Year, Manufacturer) VALUES (“E-Type”, 1970, “Jaguar”)

 The first command inserts values into all columns in the correct order. The second command inserts only some values in the wrong order, so must list columns.

www.pmt.education

Wildcards Wildcards can be used in SQL commands to specify any possible value . For example, rather than selecting a specific attribute in a SELECT command, a wildcard could be used to return all attributes. In SQL, wildcards are usually notated with an asterix. For example, using the original Cars table from before the delete command: SELECT * FROM Cars WHERE Price > 4000 >> [Polo, Volkswagen, 4995, 2010, TRUE], [Hyundai, 5225, 2013, FALSE]  The DELETE command is a bit of a special case when it comes to wildcards. The commands DELETE FROM Cars and DELETE * FROM Cars would do the same job of deleting all entries in the Cars table.

Defining a table with SQL SQL can be used to make new database tables with the CREATE command. This command specifies the name of the new table, its attributes and their data types. Also specified are entity identifiers like primary and secondary keys. For example, the following command could be used to make a table called Artists with the attributes Title, Artist and Date with a composite primary key composed of the attributes Title and Artist. CREATE TABLE Artworks (Title VARCHAR(225), Artist VARCHAR(255), Date YEAR, PRIMARY KEY (Title, Artist))  This creates an empty table. New records can be added using the INSERT command.  

Table: Artworks

Title 🔑 Artist 🔑 Date

The Night Watch Rembrandt 1642

The Persistence of Memory Salvador Dali 1931

The Great Wave off Kanagawa Hokusai 1830     

www.pmt.education

SQL Data Types Data types for attributes are specified when using the CREATE command. The data types supported by SQL are listed in the table below.

Data type SQL Description

Fixed length string

CHAR(size)  A string with the number of characters specified by size

Variable length string

VARCHAR(size)  A string with any number of characters up to the number specified by size

Integer INT(size)  A whole number stored using the number of bits specified by size

Number with fractional part

FLOAT(size, precision)  A number stored using the number of bits specified by size with digits after the decimal point up to the number specified by precision

Date DATE  A date in the format YYYY-MM-DD

Date and time DATETIME  A date and time combined in the format YYYY-MM-DD HH:MM:SS

Time TIME  A time in the format HH:MM:SS

Year YEAR  A year in one of the two formats YY or YYYY

Client server databases

A client server database system provides simultaneous access to a database for multiple clients. For example, social media websites store information on databases that are continuously being accessed and modified by different users simultaneously. Issues rarely arise when two users are requesting access to different, unrelated fields in a database. However, when different users attempt to access the same field at the same time, a problem known as concurrent access occurs. Concurrent access can result in database updates being lost if two users edit a record at the same time and can be managed with the use of record locks, serialisation, timestamp ordering and commitment ordering.

www.pmt.education

Record locks When a record is accessed by one user, it is immediately locked to other users until the first user has finished using it. Other users are blocked from accessing or modifying the content of a field until it has been unlocked. Serialisation Rather than locking a field, requests from other users are placed in a queue. Once the first user has finished using the field, the next command in the queue is executed and so on. Timestamp ordering When multiple commands are sent to the same field in a database, each is assigned a timestamp which marks the point in time at which the command was initiated. Commands are carried out on the field in the order of their timestamps. Commitment ordering When a database uses commitment ordering, an algorithm is used to work out an optimum order in which to execute commands for the same field. This algorithm will take into account the impact of commands on other parts of the database and attempt to minimise issues from occurring with the database.

www.pmt.education